Ceria based slurry for chemical-mechanical polishing

ABSTRACT

A ceria based abrasive is used in a chemical mechanical polishing operation at low polish pressure, and a predetermined pH range, to achieve high polish rates and good uniformity when planarizing layers formed from low dielectric constant materials, including but not limited to polymers. The distribution of ceria particle sizes in an exemplary slurry is bimodal and controlled.  
     In a particular embodiment a polishing abrasive containing a controlled distribution of ceria particle sizes is used in a CMP polisher apparatus with a polishing pressure of approximately 3 psi and a pH of approximately 10.6 to planarize polymer films.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of planarization, andmore specifically, to ceria based abrasives for chemical-mechanicalpolishing (CMP).

[0003] 2. Background

[0004] Advances in semiconductor manufacturing technology have led tothe development of integrated circuits having multiple levels ofinterconnect. In such an integrated circuit, patterned conductivematerial on one interconnect level is electrically insulated frompatterned conductive material on another interconnect level by films ofmaterial such as silicon dioxide. Connections between the conductivematerial at the various interconnect levels are made by forming openingsin the insulating layers and providing an electrically conductivestructure such that the patterned conductive material from differentinterconnect levels are brought into electrical contact with each other.These electrically conductive structures are often referred to ascontacts or vias.

[0005] A consequence of having multiple layers of patterned conductivematerial separated by an insulating layer is the formation of undesiredcapacitors. The parasitic capacitance between patterned conductivematerial, or more simply, interconnects, separated by insulatingmaterial on microelectronic devices contributes to effects such as RCdelay, power dissipation, and capacitively coupled signals, also knownas cross-talk.

[0006] One way to reduce the unwanted capacitance between theinterconnects is to use an insulating material with a lower dielectricconstant. Recently, polymers have been introduced for use in integratedcircuit manufacturing as dielectrics having a lower dielectric constantthan conventionally used oxides of silicon.

[0007] Nonplanar surfaces, when present in integrated circuits havingcomplex, high density multilevel interconnections, may cause the opticalresolution of photolithographic processing steps to be poor, which couldinhibit the printing of high density lines. Another problem that may becaused by nonplanar surface topography relates to step coverage of metallayers. If steps are too high or uneven, open circuits could be created.It is thus important, when making such complex integrated circuits, toplanarize the surface of many of the layers that make up the device.

[0008] Various techniques have been developed to planarize certainlayers formed during the process of making integrated circuits. In oneapproach, known as chemical-mechanical polishing, protruding steps, suchas those that may be formed along the upper surface of interlayerdielectrics (“ILDs”), are removed by polishing. Chemical-mechanicalpolishing may also be used to planarize conformally deposited metallayers to form planar plugs or vias.

[0009] Accordingly, there is a need for CMP methods and apparatus topolish low dielectric constant materials such as polymers.

SUMMARY OF THE INVENTION

[0010] Briefly, a ceria based slurry is used in a chemical mechanicalpolishing operation at low polish pressure, and a predetermined pHrange, to achieve high polish rates and good uniformity when planarizingfilms formed from low dielectric constant materials, such as polymers.

[0011] In a further aspect of the present invention, the distribution ofceria particle sizes in a slurry is bimodal, that is, controlled suchthat there are two distinct particle size ranges.

[0012] In a particular, exemplary embodiment of the present invention, aceria based polishing slurry containing a bimodal distribution of ceriaparticle sizes is used in a CMP polisher apparatus with a polishingpressure of approximately 3 psi and a pH of approximately 10.6 toplanarize fluorinated organic polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a cross-sectional view of a CMP apparatus using a convexsolid to bend a wafer, and a conventional retaining ring.

[0014]FIG. 2 is a top view of a polishing pad covered platen, retainingring, and slurry delivery arm, indicating the slurry delivery point tothe polishing pad.

[0015]FIG. 3 is a flowchart showing the operations in a process ofmaking a slurry in accordance with the present invention.

[0016]FIG. 4 is a flowchart showing the operations in a process ofpolishing a thin film in accordance with the present invention.

DETAILED DESCRIPTION

[0017] An improved method and apparatus for the chemical-mechanicalpolishing of thin films formed on a substrate is described. In thefollowing description numerous specific details are set forth to providean understanding of the present invention. It will be apparent, however,to those skilled in the art and having the benefit of this disclosure,that the present invention may be practiced with apparatus and processesthat vary from those specified here.

[0018] Terminology

[0019] Substrate, as used herein, refers to the physical object that isto be planarized by means of the CMP process. A substrate may also bereferred to as a wafer. Wafers, may be made of semiconducting,non-semiconducting, or combinations of semiconducting andnon-semiconducting materials. Silicon wafers may have thin films ofvarious materials formed upon them. These thin films may be planarizedwith CMP processing. Other substrate materials such as GaAs,silicon-on-sapphire, or silicon on insulator (SOI) may be planarizedwith CMP processing.

[0020] The terms contact and via, both refer to structures forelectrical connection of conductors from different interconnect levels.These terms are sometimes used in the art to describe both an opening inan insulator in which the structure will be completed, and the completedstructure itself. For purposes of this disclosure contact and via referto the completed structure.

[0021] The expression, low dielectric constant material, refers tomaterials having a lower dielectric constant than oxides of silicon.

[0022] The term vertical, as used herein, means substantially orthogonalto the surface of a substrate.

[0023] RPM (also rpm) refers to revolutions per minute.

[0024] Overview

[0025] Ceria is an oxide of the rare earth element Cerium (Ce) and isoften referred to by the chemical formula CeO₂. Ceria has been used forglass polishing applications such as lens and mirror manufacturing inthe past. Ceria is typically mined, and is commercially available as apowder wherein the particles have rough edges.

[0026] Polishing with relatively large, rough-edged ceria particlesprovides a fast polish, that is, a high polishing rate, but results inscratches on the surface being polished. On the other hand, polishingwith relatively small ceria particles reduces the scratching problem butalso reduces the polish rate to such an extent that polishing with thesesmall particles is impractical. In some cases of polishing with smallceria particles the polishing process is observed to “shut down”, thatis, to stop polishing.

[0027] In accordance with the present invention, organic polymerdielectrics are subjected to chemical mechanical polishing operationsusing a slurry that contains a predetermined ratio of ceria particlesizes.

[0028] Polymer films, such as those used to form low dielectric constantinsulating layers in ICs, are much softer than the oxides of siliconthat have been commonly used to form inter-layer dielectrics. When thesepolymer films are polished in a chemical mechanical polishing operation,with conventional abrasives such as silica and alumina, significantscratching of the polymer film occurs as a result of the softness of thefilm. Additionally, these organic polymer films tend to be somewhatresistant to chemical attack from water based slurry chemistries.

[0029] A slurry in accordance with the present invention is based onceria rather than the conventional abrasives such as silica or alumina.A ceria based slurry according to the present invention enables highpolish rates and substantially reduces scratching of polymer films.

[0030] The Slurry

[0031] A slurry, in accordance with the present invention, for chemicalmechanical polishing of thin films, such as polymer dielectric layers onmicroelectronic devices, includes ceria particles of at least two sizeranges, silica particles, water, and a compound for determining the pHof the slurry.

[0032] An example of a slurry in accordance with the present inventionincludes 1 part of a pre-mixed slurry, (CABOT EP7391-100 (2 wt. %silica, 6.5 wt % small particle ceria, potassium hydroxide andde-ionized water)), 7 parts de-ionized water, 2 grams/liter largeparticle ceria, 4 grams/liter potassium carbonate, and 1 gram/literpotassium bicarbonate. Alternatively, large particle ceria can be addedin amounts as low as 0.75 grams/liter.

[0033] The potassium carbonate and potassium bicarbonate act as pHbuffers. This exemplary slurry has a pH of approximately 10.6 and aspecific gravity of approximately 1.015. Those skilled in the art willrecognize that the slurry may be buffered to other values of pH.

[0034] Those skilled in the art and having the benefit of thisdisclosure will further recognize that it is not necessary to usepre-mixed slurry formulations as part of the present invention. Rather,this example is provided to illustrate the present invention.

[0035] Small particle ceria refers to crushed ceria that is filtered toa size of approximately 0.43 microns, such that approximately 99% of theparticles are of this size. Large particle ceria refers to crushed ceriathat is filtered to a size of approximately 2.8 microns, such thatapproximately 99% of the particles are of this size. The large ceriaparticles are typically added to the small particle slurry and mixedwith a motor driven stirrer. By combining the materials described above,a slurry is obtained having a predetermined ratio between large andsmall ceria particles.

[0036] Method

[0037] An embodiment of the method of polishing a thin film on a wafercontaining microelectronic devices, in accordance with the presentinvention is described in conjunction with FIGS. 1-3.

[0038] In a typical CMP system, as shown in FIG. 1, a wafer 102 isplaced face down on a rotating table 104 covered with a polishing pad106, which has been coated with a slurry 108. A carrier 100, which maybe made of a thick nonflexible metal plate 114 that is attached to arotatable shaft 105, is used to apply a downward force against thebackside of wafer 102. A retaining ring 117 may be used to center wafer102 onto carrier 100 and to prevent wafer 102 from slipping laterally.Typically, the surface of wafer 102 extends outwardly beyond thepolishing side surface of retaining ring 117. A resilient carrier pad112 positioned between metal plate 114 and wafer 102, is typically usedto press against the backside of wafer 102. Often, plate 114 will bemanufactured with a slight convex curvature so as to bend the centralportion of a wafer outward. By applying the downward force, and rotatingwafer 102, while simultaneously rotating slurry covered pad 106 for aselected amount of time, a desired amount of material may be removedfrom the upper surface of a thin film such that the surface of wafer 102is planarized.

[0039]FIG. 2 provides a top view of a conventional CMP system, showingpolishing pad 202, retaining ring 204, and slurry delivery arm 206. Theceria based slurry of the present invention is typically delivered tothe polishing pad through the use of peristaltic pumps.

[0040] Although polishing typically takes place in a range oftemperatures from about 10° C. to about 25° C., further chilling thepad, slurry, or wafer may improve polymer polishing results. Generally,hard materials polish more easily than soft materials. Polymers tend tobe softer than other materials that are routinely planarized by chemicalmechanical polishing. Since polymers tend to harden with decreasingtemperatures, chilling the polymer film on the wafer will tend toprovide a harder surface for polishing.

[0041] Although polymer films are described in the illustrativeembodiment herein, the ceria based slurry in accordance with the presentinvention is suitable for polishing any low dielectric constant thinfilm, including but not limited to fluorine doped oxides of silicon,amorphous fluorinated carbon, parylenes, and PAE (para arol ethylene).Organic polymers used for low dielectric constant insulators inmicroelectronic devices can be doped with fluorine to further reducetheir dielectric constant, and these can also be polished with the ceriabased slurry of the present invention.

[0042]FIG. 3 shows a flow diagram of a process 400 embodying the presentinvention. At block 402, a slurry containing two distinct size ranges ofceria is prepared by mixing together large size and small size ceriaparticles. An exemplary formulation of a slurry in accordance with thepresent invention is described above in the preceding section. Theslurry is then delivered to the polishing pad as shown at block 404.Then, as shown at block 406, a film is polished as a wafer coated withthe thin film is brought into contact with a rotating polishing pad ontowhich the slurry containing large and small ceria particles has beendelivered.

[0043]FIG. 4 shows a flow diagram of an alternative process 500embodying the present invention. At block 502, a ceria containing slurryis prepared. As shown at block 504, the ceria containing slurry isdelivered to, and dispensed onto, a polishing pad. Then, as shown atblock 506, an organic polymer film is polished as a wafer coated withthe thin organic polymer film is brought into contact with a rotatingpolishing pad onto which the slurry containing ceria has been delivered.Typical polishing conditions are down force of approximately 3 psi, 180rpm effective rotational speed, temperature between approximately 10° C.and 25° C. Polishing pads such as the Politex Supreme, and Suba-4, madeby Rodel, Inc. of Newark, Del., can be used with the ceria based slurryto polish organic films. Removal rates of greater 2250 angstroms/minutehave been obtained when polishing organic material such as parylene.

[0044] Conclusion

[0045] Embodiments of the present invention provide ceria based slurriesfor chemical mechanical polishing of thin films, at low pressure andhigh polish rates. These thin films are typically comprised of materialsthat are softer than silicon dioxide films. For example, organic polymerthin films are softer than silicon dioxide thin films.

[0046] An advantage of some embodiments of the present invention is thatthe polish time can be reduced thus reducing manufacturing cost.

[0047] A further advantage of some embodiments of the present inventionis that dishing is reduced when polishing polymer films.

[0048] A still further advantage of some embodiments of the presentinvention is that scratching of polymer films during polishing isreduced.

[0049] It will be apparent to those skilled in the art a number ofvariations or modifications may be made to the illustrative embodimentsdescribed above. For example, various combinations of particle sizes,slurry pH, slurry delivery rate, pad rotation speed, pad temperature,and so on, may be used within the scope of the present invention.

[0050] Other modifications from the specifically described apparatus,slurry, and process will be apparent to those skilled in the art andhaving the benefit of this disclosure. Accordingly, it is intended thatall such modifications and alterations be considered as within thespirit and scope of the invention as defined by the subjoined claims.

What is claimed is:
 1. A method of planarizing a film, comprising:polishing the film with a ceria based slurry; wherein the film comprisesan organic polymer.
 2. The method of claim 1, wherein the organicpolymer comprises a fluorinated polymer.
 3. The method of claim 1,wherein the organic polymer is selected from the group consisting ofparylene, and para arol ethylene.
 4. The method of claim 1, wherein theceria based slurry comprises ceria particles having a first size, ceriaparticles having a second size, and wherein there is a predeterminedratio of the ceria having the first size particles to the ceria havingthe second size particles.
 5. The method of claim 4, wherein the firstsize is approximately 0.43 microns and the second size is approximately2.8 microns.
 6. The method of claim 1, wherein the slurry containsapproximately 1% to 3% by weight of ceria.
 7. The method of claim 1,wherein the slurry contains approximately 1.5% by weight of ceria. 8.The method of claim 1, wherein polishing comprises chemical mechanicalpolishing with a down force of less than approximately 4 psi.
 9. Themethod of claim 1, wherein polishing comprises: engaging the film with apolishing pad with a down force of approximately 3 psi; and providing aneffective rotational speed of approximately 180 rpm.
 10. The method ofclaim 1, further comprising controlling the temperature of the film suchthat polishing occurs in the range of approximately 10° C. to 25° C. 11.A method of planarizing a film, comprising: polishing the film with aceria based slurry; wherein the ceria based slurry comprises a firstamount of ceria particles having a first size range, and a second amountof ceria particles having a second size range, wherein there is apredetermined ratio of the first amount to the second amount.
 12. Themethod of claim 11, wherein polishing comprises bringing the film intocontact with a rotating polishing pad.
 13. The method of claim 11,wherein the first size range requires approximately 99% of the particlesto be filtered to 0.43 microns and the second size range requiresapproximately 99% of the particles to be filtered to approximately 2.8microns.
 14. The method of claim 11, wherein the predetermined ratio ofceria particles sizes is selected to prevent shut down.
 15. A slurry,comprising: a first oxide of cerium having a particle size range whereinapproximately 99% are approximately 0.43 microns; and a second oxide ofcerium having a particle size range wherein approximately 99% areapproximately 2.8 microns.
 16. The slurry of claim 15, furthercomprising an oxide of silicon.
 17. The slurry of claim 15, furthercomprising water.
 18. The slurry of claim 17, wherein the water isdeionized.
 19. The slurry of claim 15, further comprising: silica;potassium hydroxide, potassium carbonate, potassium bicarbonate, anddeionized water; wherein the concentration of the first oxide of ceriumtogether with the second oxide of cerium is in the range ofapproximately 1.0% to 3.0% by weight; wherein the concentration of thesilica is approximately 0.5% by weight; and wherein the slurry has a pHof approximately 10.6.
 20. A slurry comprising: ceria having a firstrange of particle sizes; ceria having a second range of particle sizes;silica; potassium hydroxide; and de-ionized water; wherein theconcentration of the ceria having the first range of particle sizes,together with the ceria having the second range of particle sizes isapproximately 1.5% by weight; and the concentration of silica isapproximately 0.5% by weight.
 21. The slurry of claim 19 furthercomprising a pH buffer.
 22. The slurry of claim 21, wherein the pHbuffer comprises potassium carbonate and potassium bicarbonate.
 23. Aslurry for polishing an organic polymer films on a wafer, comprising: 1part of a pre-mixed slurry, comprising 2 wt. % silica, 1.5 wt % smallparticle ceria, potassium hydroxide and de-ionized water; and 7 partsde-ionized water, 2 grams/liter large particle ceria, 4 grams/literpotassium carbonate, and 1 gram/liter potassium bicarbonate; wherein theslurry has a pH of approximately 10.6 and a specific gravity ofapproximately 1.015.